It's trite to say a major crisis or tragedy changed everything. But the Boston Marathon bombing occurred smack in the middle of an extraordinarily active political season. Some big initiatives suffered setbacks in its wake, and a few new ones -- welcome back, national-security politics, and we can't wait to learn how Chechnya matters to us now -- are suddenly urgent. Here's some of what the ABC News political unit will be tracking this week.

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Immigration Nation

That wasn't quite the rollout they had in mind. Comprehensive immigration reform was supposed to be the big story this week, with the Senate's long-awaited "Gang of Eight" compromise unveiled to an expectant public. But then the busiest news week approximately ever happened, and the first big push on immigration reform was completely lost. Even worse, potentially, were early indications from some opponents of the reform bill about a willingness to link the events in Boston to questions of border security and immigration policy.

Sen. Chuck Grassley, R-Iowa, suggested Friday that the investigation of two alleged attackers who were immigrants will "help shed light on the weaknesses of our system." Ann Coulter was more explicit -- if not outright offensive: "It's too bad Suspect # 1 won't be able to be legalized by Marco Rubio, now," she Tweeted. Senators John McCain and Lindsey Graham responded to those arguments in a statement saying Boston demonstrates the need for immigration reform: "Immigration reform will strengthen our nation's security by helping us identify exactly who has entered our country and who has left."

President Obama suffered perhaps the biggest setback of his presidency this past week with the failure of the Senate to move forward on the bipartisan background checks proposal. Senate Democratic leaders were forced to pull the entire gun bill, and they won't bring it up again unless the political dynamics shift considerably in their favor.

It's a test for the Obama grassroots operations, Organizing for Action, as supporters of new gun controls hope to harness the emotion of gun victims' families into changing some votes. Liberal groups are making noises about challenging the four Democrats who voted against the background-check amendment, and the Gabrielle Giffords and Michael Bloomberg operations are also pledging to stay in the game, potentially targeting members of both parties. Realistically, though, major new gun controls are effectively done.

Thursday brings the dedication ceremony of the George W. Bush Presidential Center, at Southern Methodist University in Dallas. That means a round of reflection and reexamination of the legacy of the 43rd president of the United States, more than four years after he left office. It's been a quiet period for the former president, with him choosing to stay almost entirely off the political stage, and with even Republicans continuing to have mixed-at-best feelings about his eight years in office. Expect a slightly more active phase for Bush now, through the library and museum and the George W. Bush Institute. Thursday morning's ceremony also will mark an extremely rare occasion for the current president and all three living former presidents (Carter, George H.W. Bush, and Clinton) to convene together.

Just when you thought the Mark Sanford-Elizabeth Colbert Busch House race in South Carolina couldn't get any weirder, the Appalachian Trail led to ... the Super Bowl? News that Sanford's ex-wife, Jenny, has filed multiple complaints against her husband in connection with their separation agreement has ricocheted through the race. National Republicans pulled their support for Sanford when news broke of a trespassing complaint, and Sanford didn't make it much better with a strange explanation that involved not wanting his 14-year-old to have to watch the Super Bowl without his father present.

A growing number of Republicans have come to the conclusion that it may be better to write off the Charleston-area district for a year and a half (it's heavily Republican, and anybody other than Sanford would be favored to win it next fall) than continue to deal with Sanford's erratic behavior. One conservative group is even planning a write-in campaign for Jenny Sanford. That means that Stephen Colbert's sister could be coming to Congress. National Democrats are now investing in the race, with some estimates of spending in the neighborhood of $1 million and counting. The special election is May 7.

In time for the scattered security scares in cities across the country ... the Federal Aviation Administration begins its furloughs of 47,000 employees, starting Sunday. The Department of Transportation is warning of delays of more than two hours outside of what would be normal weather delays as a result of the reduced manpower; one official said it will be "the equivalent of weather events all around the country all at once." The airports expected to be most affected -- Newark, JFK, LaGuardia, O'Hare, LAX, Atlanta -- are hardly models of efficiency under the best of circumstances. So this could get ugly, potentially making those pesky automatic budget cuts known as the sequester a hot political topic again.